Microdischarge detectors (MDDs) or sensors may be used for detecting the presence of molecules in a gas sample on the basis of their optical emission spectrum as excited and emitted by that discharge.
A microdischarge detector comprises a chamber within which a sample (e.g., a gaseous or vapor fluid) is introduced so as to flow between two closely spaced electrodes. The electrodes typically might be spaced on the order of 20-200μ apart. A voltage is generated across the electrodes so as to cause an electrical discharge between the electrodes. Typical voltages for a microdischarge detector might be on the order of 200-600 Volts. This voltage and associated current could be continuous (i.e., DC) or alternating AC. As the sample fluid passes between the electrodes and gets hit by the electrical discharge, the elemental components of the fluid will emit electromagnetic waves. Every element has a characteristic emission spectrum or signature spectrum. One or more photodetectors detect the emission spectrum. Typically, the microdischarge detector will have an array of photodiodes, each photodetector filtered to receive a different, narrow bandwidth of radiation. The emission spectrum can then be analyzed to determine what element or elements comprise the sample.
One particular scientific measurement instrument in which a microdischarge detector is used is a gas chromatograph (GC). In a gas chromatograph, a sample pulse of gas is introduced into a carrier stream of another gas. The carrier stream typically comprises helium, hydrogen, or nitrogen. However, other carrier gases, such as air (environmental or a patient's breath) may also serve as a carrier gas, especially in a micro gas chromatograph, such as PHASED (see, e.g., U.S. Pat. No. 6,393,894).
A pump pushes or pulls the carrier gas through a tortuous capillary path containing a polymer that adsorbs and desorbs the molecules of the gases. The polymer, for instance, may be a coating on the internal walls of the capillary path. The sample gas whose composition is to be determined is introduced as a pulse into the carrier gas at the inlet to the capillary path. The polymer coating adsorbs and desorbs the molecules in the gas mixture (including the molecules of the carrier gas as well as the molecules of the sample pulse gas). The heavier the molecule within the mixture, the more slowly is it adsorbed and desorbed. Accordingly, the heavier the molecule, the longer it will take to pass through the capillary from the inlet to the outlet. The outlet of the capillary is connected to a microdischarge detector. The microdischarge detector, therefore, detects not only the electromagnetic emission spectrum as peaks pass through the electrodes, but also the time at which the peak passes through the electrodes. Accordingly, the output information from the microdischarge detector provides two dimensions of data that can be used to determine what atoms and/or molecules are in the sample gas, namely, 1) the time delay through the capillary for each peak peak, and 2) the emission spectrum of each peak.
Since polymer adsorbs and desorbs the carrier gas also, the signature emission spectrum of the carrier gas also is detected by the microdischarge detector. This background signal, i.e., the electromagnetic emission lines/bands and/or electrical plasma properties of the carrier gas essentially constitute interference with the measurement of the analytes to be detected and carried by the sample gas. This is particularly problematic if the carrier or sample gas is air since air is a mixture containing N2, O2, H2O, Ar, CO2, NOx and additional trace gases, all of which have emission spectra. The emission bands of all of these molecules may mask the sample gas microdischarge emission properties and “signatures” of the analytes of interest. They also can change as a function of pressure, time and temperature, further diminishing the ability to obtain an accurate measurement.
Conventionally, zero spectral emission between the active emission bands is used as a reference baseline in spectrometers. As an improvement of that, commercial spectrometers, such as those by Ocen Optics, provide means for subtracting the carrier gas spectrum from the sample gas or peak-gas emission spectrum, to better visualize and measure the bands of interest. However, such spectrometers record the two spectra sequentially in time before subtracting one from the other.
Accordingly, it is an object of the present invention to provide an improved microdischarge detector system.
It is another object of the present invention to provide an improved gas chromatograph.